The present invention relates in general to a controller for a milling machine and pertains, more particularly, to a controller for automating a milling machine so as to provide accurate reproducability of parts. The controller enables operation in basically two modes including a program mode in which the machinist by going through a series of machining steps essentially records a program, and a run mode in which the previously recorded program is used for automatic control of the machine. The controller of this invention is adapted to provide operation substantially identically on part after part with a repeatability accuracy of on the order of + or -0.001 inch. Furthermore, although the system incorporates a microprocessor, it can be operated by an ordinary machinist without requiring any knowledge of digital information processing.
Programmable milling machines presently available are quite costly, whereas the device of the present invention can be constructed for only a fraction of the cost of existing machines and can be used to convert a milling machine such as a Bridgeport miller into an automated machine. Furthermore, with the present invention, the operation is much easier without requiring a skilled programmer.
One of the main reasons as to why existing machines are relatively expensive is because they operate on the premise of completely eliminating backlash between the lead screw that drives the table and the nut or similar device fixed to the milling machine. A normal milling machine, even when new has some looseness or play which makes programming very difficult by means of a normal digital keyboard input. Assuming that one turns the handcrank attached to the lead screw on the moveable table in one direction, for each rotation of the crank the table moves approximately 0.200 inches. In reversing the direction of rotation, because of necessary tolerances and because of wear, the table generally will not move for from 1/8 to 1/4 of a turn. Since most of the work is accomplished with the work near the center of the table, the wear is greater near the center of the lead screw than at the ends. Therefore, there might be a backlash of say 0.048 inch at one point and perhaps 0.019 inch near the ends of the screw. Because existing numerical control machines operate on the premise of feeding digital information to the milling machine from a program, there is no practical way to take this variation into account, particularly since one usually desires an accuracy on the order of + or -0.001 inch. For this reason, programmable milling machines try to completely eliminate backlash through extremely expensive techniques such as the use of pre-loaded ball screws. Furthermore, machines of this sort must be programmed by a skilled programmer who must interpret a drawing into numerical language usually using a device similar to a typewriter in constructing a program paper tape to input to the microprocessor. Punched paper tape is most commonly used but magnetic computer tape may also be used. Normally, the average machinist is not capable of operating such a highly sophisticated programmable machine.
Moreover, the completed program with such existing machines is entered into a reader on the milling machine and this requires a skilled set-up man. This means that man and machine time and possibly some parts are sacrificed while setting up the machine.
Accordingly, it is an object of the present invention to provide an improved controller for a milling machine or the like that is of much simpler construction and which eliminates the need for taking into account backlash and similar tolerance problems inherent in a milling machine.
Another object of the present invention is to provide an improved controller for a milling machine which provides for the accurate reproducability of parts.
Still another object of the present invention is to provide an improved controller for a milling machine or the like which in particular can be operated by the normal machinist without requiring the use of a skilled programmer or skilled set-up man.
Still a further object of the present invention is to provide an improved controller for a milling machine that has an easily useable control panel and which is adapted to provide for control for all axes of operation, particularly X and Y operation.
Still another object of the present invention is to provide an improved controller for a milling machine including improved mechanical coupling to the lead screw which controls the machine table.